'Can', 'Skilled', 'Head Shots'...etc etc lots of itf, buts, shoulds and maybes!!!
Of course a .223 can kill deer but can you do so consistently? Raise your hand if you have ever made a less than stellar, marginal shot on a deer....
I find these statements about only taking head shots and being an amazing marksman with a grain of salt.
Yes a marginal lung hit with .223 will kill you deer but the same hit with bigger bullets will do it more quickly.
Blood trail...I have shot African Impala with a .223 and 55gr rem factory loads. Impala are smaller than most deer, have hardly any sun-cutaneous fat and thin hair, deer dont! Those impala didn't bleed much at all, making a potential 100yrd follow up in bush difficult! Yes the .223 killed them well but one ram I pulled the shot high and hit the spine which put him down but he was not dead as the little bullet did not break it....even a .243 would have been much more effective.